U.S. patent number 3,787,752 [Application Number 05/276,017] was granted by the patent office on 1974-01-22 for intensity control for light-emitting diode display.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Dennis G. Delay.
United States Patent |
3,787,752 |
Delay |
January 22, 1974 |
INTENSITY CONTROL FOR LIGHT-EMITTING DIODE DISPLAY
Abstract
A plurality of light-emitting diode elements which are unmatched
in light tput at the lower portions of their forward current ranges
and which form an integrated illuminated visual display are
activated by a power supply arrangement which applies to each diode
element a series of independent power pulses of sufficient power to
activate each diode element to saturation and into light-emitting
condition in a frequency range which appears to the human eye to be
steady illumination, the power supply arrangement being provided
with a control feature for selectively varying the duration of the
power pulses in order to vary the apparent intensity of the
illuminated display, and also provided with current limiting means
to limit the current passed by each diode element, during each
power pulse and its activation thereby, to a given upper range of
forward current in which the light-emitting diode element light
outputs are substantially matched and in which optimum light
emission efficiency is achieved.
Inventors: |
Delay; Dennis G. (Oxnard,
CA) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
23054802 |
Appl.
No.: |
05/276,017 |
Filed: |
July 28, 1972 |
Current U.S.
Class: |
327/544; 968/940;
345/39; 345/34; 327/318; 345/691 |
Current CPC
Class: |
G04G
9/0088 (20130101); G01R 13/405 (20130101) |
Current International
Class: |
G01R
13/00 (20060101); G01R 13/40 (20060101); G04G
9/00 (20060101); H03k 019/14 () |
Field of
Search: |
;315/84.5,84.6,169R,169TV ;313/18D ;340/336 ;307/228,311 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Mullins; James B.
Attorney, Agent or Firm: Sciascia; R. S. Hansen; Henry
Braddock; H. E.
Claims
1. An improved solid state electrical instrument system for
achieving uniform intensity of an illuminated visual display
arrangement, comprising in combination:
a plurality of light-emitting diode elements arranged in the visual
display arrangement, said elements being unmatched in light output
over the lower portions of their forward current ranges and matched
at a given upper portion of the forward current ranges;
a power supply means for generating a continuous series of
independent electrical power pulses at a frequency above which said
diode elements when activated appear to the human eye to be
continuously illuminated, each of said pulses being of more than
sufficient power to activate said diode elements in their given
upper portions of their forward current ranges, and including
control means for selectively varying the duration of the power
pulses to vary the apparent intensity of the visual display
arrangement formed by said diode elements; and
forward current limiting means connected to receive said pulses for
limiting the forward current through the diode elements on each
power pulse to a predetermined given upper portion of their forward
current range in which the light outputs of the light-emitting
diode elements are
2. The improved system of claim 1 wherein said forward current
limiting means further comprises:
divider means receiving and biasing said pulses; and
regulating means receiving said biased pulses and for regulating
the current output of said pulse within the predetermined portion
of the
3. The improved system of claim 2 wherein said divider means
comprises:
a regulated voltage source;
a first resistor connected at one terminal to the output of said
power supply means; and
a second resistor connected between said regulated voltage source
and the
4. The improved system of claim 3 wherein said regulating means
comprises:
a transistor having an emitter connected to said regulated voltage
source, a base connected to said other terminal of said first
resistor, and a collector connected to said diode elements; and
capacitor means connected between said regulated voltage source and
ground.
5. The improved system of claim 4 wherein said power supply means
further comprises:
a sawtooth oscillator circuit means for producing a sawtooth
wave;
a differential comparator unit receiving the sawtooth wave and a
selected DC reference voltage from said circuit means for producing
an output pulse
6. The improved system of claim 5 wherein said control means
includes a selectively adjustable means for varying the level of
the selected DC reference voltage to said comparator unit.
Description
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or
for the Government of the United States of America for governmental
purposes without the payment of any royalties thereon or
therefor.
BACKGROUND OF THE INVENTION
This invention relates generally to the field of selective
intensity control systems for illuminated electrical displays, and
more specifically to an improved intensity control arrangement for
uniformly controlling the apparent intensity of a plurality of
light-emitting diode elements forming a visual display in an
airborne instrument system by the use of a duration-modulated power
pulse system.
Duration-modulated pulses have been used generally in the known
prior art to drive light-emitting gas discharge tubes, such as
nixie tubes. The use of light-emitting diodes to form visual
displays generally is known in the prior art. However these prior
art arrangements apparently have not recognized the problems in
specifically applying light-emitting diodes to illuminated visual
displays requiring high efficiency and effective precise controls
such as those used in high speed aircraft especially those intended
for military use. It has been determined to be difficult and costly
to produce light-emitting diodes which are closely matched over the
entire range of their forward operating current. In addition it has
been found necessary to drive these diodes essentially to the
saturation point to achieve the desired illumination. In aircraft
installation, space, weight, and available power are at a premium
and build-up of undesirable amounts of generated heat by electrical
units must be minimized. It can be seen that successful application
of alight-emitting diodes with their desirable illuminating
capabilities to airborne displays requires careful design to solve
the related special technical problems involved. The prior art does
not appear to have identified these problems, or provided variable
intensity instrument display systems which could satisfactorily
cope with them, or utilize the desirable features of recent
light-emitting diode technology.
SUMMARY OF THE INVENTION
The shortcomings of the prior art visual display systems have been
overcome and the hereinafter-mentioned objects of the invention
have been achieved by the improved intensity control system for
airborne display instruments making most effective use of the
desirable light-emitting diodes. This system is an improved solid
state system for achieving uniform intensity control of an
illuminated visual display formed by a plurality of light-emitting
diode elements which are unmatched in light output over the lower
portions of their forward current ranges and matched at a given
upper portion of their forward current ranges, said system
comprising in combination: a plurality of light-emitting diode
elements arranged in a visual display arrangement, said elements
being unmatched in light output over the lower portions of their
forward current ranges and matched at a given upper portion of
their forward current ranges; a power supply means operatively
connected to activate the diode elements to a light-emitting state
by generating and applying to each of said diode elements a
continuous series of independent electrical power pulses at a
frequency above which said diode elements when activated appear to
the human eye to be continuously illuminated, each of said pulses
being of variable duration and of more than sufficient power to
activate said diode elements in their given upper portions of their
forward current ranges, said power supply means further comprising
control means for selectively varying the duration of the power
pulses to vary the apparent intensity of the visual display
arrangement formed by said diode elements, said system further
comprising; forward current limiting means cooperating with said
diode elements and said power supply means to limit forward current
through the diode elements on each power pulse to a predetermined
given upper portion of their forward current range in which the
light outputs of the light-emitting diode elements are
substantially evenly matched.
STATEMENT OF THE OBJECTS OF THE INVENTION
It is an object of the invention to provide a novel improved
intensity control system for illuminated visual displays of special
advantage for aircraft applications, a system which overcomes the
deficiencies of the prior art systems and takes advantage of the
highly desirable properties of light-emitting diode technology to
improve control, uniformity of illumination, and effectiveness,
reduce size, weight, power consumption and cost, and provide such a
system which is simple in construction, easy to fabricate, operate,
and service, yet rugged and reliable in operation for long periods
of operating life.
Other objects and advantages will become apparent from a
consideration of the following specification, the claims, and the
accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a general schematic or functional block diagram of an
aircraft engine RPM counting and indicating instrument system with
a selective illumination intensity control arrangement embodying
features of this invention.
FIG. 2 is a partial perspective view of a basic two-engine aircraft
tachometer unit with an intensity control system for the RPM
display embodying principles of this invention. Certain parts are
broken away and others shown partially disassembled for a clearer
showing of features and their locations.
FIG. 3 is a partial vertical cross-sectional view through the front
display face portion of the unit of FIG. 1 showing the general
construction and arrangement of the visual display formed by the
light-emitting diodes and the selective illumination intensity
control element.
FIG. 4 is a circuit diagram of the electrical intensity control
circuit of the invention.
FIG. 5 is a general graphical presentation illustrating the manner
in which varying the DC reference voltage input to the comparator
varies the sawtooth output bias level to control the duration of
the output pulses from the comparator.
FIG. 6 is a circuit diagram of the zener supply circuit for the
differential comparator of FIG. 5.
FIG. 7 is an illustrative showing of the preferred way in which the
light-emitting diodes for one decade of the linear bar graph
display are electrically connected between the power supply means
with its intensity control features and the encoder means drive or
switching arrangement for this display.
FIG. 8 is a graphical presentation showing the typical relationship
of forward current to forward voltage for the light-emitting diodes
preferably used in the system of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1 a tachometer or instrument system with the
intensity control feature of the invention comprises a display face
assembly DF cooperating with an intermediate housing IH and a rear
housing RH. The housings generally contain the power supply and
circuitry components needed to activate the information display of
the display face assembly. The information display for a two-engine
aircraft consists of a vertical bar graph presentation BG for each
engine and a corresponding numeric readout or presentation N
appearing in a window 3 in housing 2 of the display face
assembly.
Engine RPM of the left and right engines is presented in increments
of five percent (from 0 to 70 percent of rates RPM) and one percent
(from 70 to 110 percent of rates RPM) on the bar graph
presentations or displays. Each bar graph display comprises 54
light-emitting diodes, indicated as LED in the drawings. Each diode
is preferably sized at about 0.020 inches by 0.030 inches and will
be described in greater detail hereinafter. Each numeric display is
made up of two seven-segment numeric indicators and a single
two-segment numeric indicator as shown in FIG. 1. The seven-segment
numeric indicators are configured in a window frame format as shown
from 14 light-emitting diodes (0.35 inches by 0.015 inches each)
with two diode elements per segment. The numeric display reads out
percent of rated engine RPM in one percent steps over the full
range of the system to provide the pilot or viewer with a precision
RPM reading at any desired time.
The diode elements of the bar graph display are configured in
decades with each decade having its own constant current source and
decade enable line (FIG. 7). The constant current sources provide
uniform light intensity from all diode elements of this
display.
The numeric display indicates the same data or information shown in
the cooperating bar graph to provide redundancy and reliability.
Generally, referring to FIG. 1, after the tachometer input signal
is clipped, limited and multiplied by a factor of four, it is
counted by the same type of decade counter as the bar graph display
signal. At the end of each count period a store gate occurs and new
count data is transferred to storage registers. From the storage
registers the information is processed by BCD to seven-segment
decoders which double as drivers to the light-emitting diode
elements.
Intensity of the light-emitting diode displays is controlled by
modulating the "on" currents of the diode elements by a variable
pulse width generator. This variable duty cycle control gates the
diode elements on and off at a non-flicker rate, the variable duty
cycle gating appearing to the human eye as an intensity variation
on the diode element emitted light. Since the diode elements are
very rapidly driven to upper ranges of forward current on each
power pulse, the various diode elements light emission need not be
matched over the forward current operating curve but only at the
predetermined upper range of this current. The diode elements are
matched as to light emission only at this upper current range which
simplifies their manufacture and lowers their cost. In addition,
the "on" state is selected to involve that value or range of
forward current where the light-emitting diode element efficiency
is optimum. The intensity control circuit will be more fully
described hereinafter.
Generally describing light-emitting diodes and their functioning,
the direct conversion of electrical energy to light energy, or
electroluminescence, is reasonably well known, having been first
observed in semiconductors in about 1923 during work with silicon
carbide detectors. The flow of current rhough the silicon carbide
produced light without the crystal becoming incandescent, the color
of the emitted light depending on the material and experimental
conditions. This technology remained rather inactive until the
1950s when workers in the field applied theories developed for p-n
junctions in transistors developed a theoretical explanation and
renewed interest in the electroluminescent properties of
semiconductors. Early light-emitting diodes radiated infrared and
visible red color and more recently brightness and efficiency have
improved to the point where they can be used to alert operating
personnel even in well-lighted environments. The light emission is
produced by injection and recombination of electrons and holes in
the crystal material. As these excess carriers recombine, they give
up energy in the form of photons.
Suitable efficient light-emitting diodes for use in systems
embodying the present invention are fabricated from gallium
aluminum arsenide using a solution growth technique. This method
permits the formation of carefully-controlled p-n junctions by
slowly cooling a solution of Ga, Al, and GaAs, where the Ga is in
excess, and n or p type dopants (Te and Zn). Epitaxial layers of
GaAlAs are deposited on a GaAs substrate as the solution cools from
about 1000.degree. C to about 880.degree. C. Later, the substrate
crystal is removed and the GaAlAs p-n junction that remains is
provided with contacts using vapor-deposited metallurgy techniques.
Through careful control of dopants and material concentrations, the
emission wavelength of the diodes may be varied from about 6000
Angstroms (visible) to 9000 Angstroms (near IR).
Light-emitting diodes require reasonably high forward current
densities before useful emission can occur. This current is
generally greater than 10 amperes/cm.sup.2. For a typical GaAlAs
diode crystal of square 0.015 inch .times. 0.015 inch
configuration, and assuming 20 milliamperes are required for a
nominal value of 400 ft-lamberts, the current density J is about 14
amperes/cm.sup.2. A forward voltage versus forward current curve
for a typical GaAlAs light-emitting diode is shown in FIG. 8.
A summary of GaAlAs light-emitting diode characteristics is
presented in Table I.
TABLE I
GaAlAs Light-emitting Diode Data
Min. Max. Units Power Dissipation Derate Linearly from 25.degree.C
2.5 mW per .degree.C 150 mw. Forward Current, Continuous 100 ma.
Peak Forward Current 1 us Pulse, 300 pulses per second 3 amps
Operation Temperature -55 100 .degree.C Reverse Voltage at
I=10.mu.A 3 volts
Electro-Optical Operating Characteristics (25.degree.C Unless
Otherwise Specified)
Min. Typical Max. Units External Radiated Power If=50 mA 3 mw. Peak
Emission Wavelength 6000 9000 A. Emission Line Half Width 375 A.
Forward Voltage Drop at If=100 mA 1.8 volts Forward Dynamic
Resistance at If=100 mA 20 ohms Reverse Current at VR=3.sup.V 1.0
ma. Capacitance at Vf=0 100 pf. Capacitance at Vf=0.8V 150 pf.
Capacitance at V.sub.R =3v 70 pf. Light Turn-on Time 25 ns. Light
Turn-off Time 25 ns.
Thermal Characteristics
Min. Typical Max. Units Wavelength Temperature 1.3 A. Coefficient
(Case per .degree.C Temperature) Forward Voltage -1.5 mv.
Temperature per .degree.C Coefficient Vf/T Output Attenuation .43 %
per .degree.C Temperature Ref. at 20.degree.C Coefficient %/T
more specifically the preferred diodes for use in the system of the
invention are GaAlAs elements approximately 35 .times. 15 mils in
size. A host crystal, serving as a substrate, of GaAs approximately
18 mils thick is the starting material. On top of this substrate a
layer of N-type GaAlAs of approximately two mils thickness is
grown. In turn, another layer of P-type GaAlAs is grown over the
N-layer. Both the substrate and the P-layer are opaque while the
N-layer is transparent with respect to the red light which is
emitted when current is passed through the final p-n junction. The
substrate is then lapped off leaving a p-n wafer approximately four
mils thick. A layer of metal is then deposited on both sides of
this wafer. The p-side is uniformly coated with gold/zinc, while
the n-side is coated with gold/germanium/nickel through a mask
which defines the N contacts. The whole wafer is then heated and
the semiconductor-metal contacts are formed by the resulting
alloying process. The metallized wafer is then cut up into discrete
diode elements by a string saw. An etching process is then carried
out to remove crystal damage caused by the sawing operation. Each
diode element has a solid metal layer on the bottom or p-side and a
metal contract on the n-side which will accommodate interconnecting
wires. Each diode element is then bonded with a conductive epoxy
compound to an anodized aluminum block. This block, which is
machined to size is then anodized to provide an aluminum oxide
insulation, and acts as a heat sink for the diode elements. The
base of block is designed to maintain the diode element array at
40.degree. C temperature when the ambient air is 25.degree. C. The
block is prepared with a black dye to eliminate reflective
surfaces. One mil gold wires are used as flying leads to make the
diode-diode and diode-terminal connections. In operation the
display becomes visible, or illuminated as a result of the light
generated within the p-n junction and passing through the top
transparent N-layer. As seen in FIG. 8 the current-voltage
characteristics of the light-emitting diodes used are similar to
ordinary semiconductor diodes in shape. As voltage increases to
about 1.8 volts the current change is somewhat gradual. However, at
this point the current begins increasing very rapidly without
appreciable voltage increase and will stress the diode/wire
interface. As will become apparent in the following description a
constant current source or series resistor in conjunction with a
constant voltage source must be used to light or activate these
diode elements. In the preferred system of the invention the
constant current approach is used for the bar graph display and the
series resistor is used for the numeric display.
As shown in FIGS. 2 and 3, the display face assembly DF contains
the light-emitting diode elements LED plus filter and other
elements for visual enhancement of the overall display. Directly
inside an opening 3 in the front of aluminum housing 2 of display
face assembly DF is a cover glass 11 which has an HEA non-glare
coating on both sides. Against this glass element is a plexiglas
edge lighting insert element 12 which is provided with a plurality
of incandescent lamps (not shown) to provide conventional
illumination to engraved legends on red filter element 13. This
filter element 13, visible from the front of the instrument is a
clear circular polarized filter and a red filter sandwiched
together. This filter over the black background of the heat sink
member HS on which the light-emitting diodes LED are mounted
provides maximum visual enhancement of the display. The diode
element array on heat sink member HS is directly behind filter
member 13. Diode elements LED are encapsulated beneath a layer of
clear epoxy material to protect the diode elements, their leads,
and interconnections. In addition the clear epoxy coating more
effectively optically couples the diode element to the ambient air,
giving an apparent brightness improvement of more than two over the
unencapsulated condition.
The intermediate housing portion IH of the instrument comprises an
aluminum framework member 1 to which is secured, by suitable
conventional means, the display face assembly DF, its diode element
heat sink member HS, two main printed circuit cards MCC, power
converters and other elements (not shown).
The instrument has been designed for easy maintenance, the display
face assembly DF, diode element array and heat sink HS being
removable from the front, and the electronic circuit cards designed
to swing out from the sides from operative positions in member 1,
with conductor bundles WB acting as hinged connections, to lie flat
on a work bench. Two rectangular cover plates having an inside
coating of a glass-epoxy compound are secured by suitable means to
the sides of framework member 1 to enclose printed circuit cards
MCC in the intermediate housing. The glass-epoxy compound provides
electrical insulation to prevent the printed circuit card
conductors from shorting out against the housing. A durable
conformal coating is applied to the component side of the circuit
cards MCC to provide a good mechanical support between the
components, wiring and the cards. Printed circuit cards are
identical except for intensity control transistor Q7 for the bar
graph.
Rear housing RH is enclosed by wall member 4 and contains
electrical power supply units PS, printed circuit clock card CC and
a fan for cooling air mounted in casing F. Wall member 4 also
supports power supply connection PI and tachometer signal input
connection TSI.
As shown by the arrows in FIGS. 2 and 3 the cooling fan in casing F
brings in cooling air through the spaces 8 between glass panel 11
and front wall 2 of the display face assembly DF, moves the cooling
air over the diode element array and heat sink HS, rearwardly
through the intermediate housing IH and out through opening 7 into
rear housing RH where it is passed over the power supply units PS
for the light-emitting diodes and exhausted out the bottom of
housing RH as shown through an exhaust opening not shown.
Clock card CC and power supply units PS can be reached for test and
service by removing housing element 4. Power supply units are
mounted on a wall portion of rear housing RH which wall portion is
provided on its exterior surface with cooling fins FI for
additional cooling affect.
Manually adjustable knob IC on the front of the display face
assembly and the cooperating intensity control potentiometer ICR
provide the means for selecting and adjusting the intensity of
light produced by the light-emitting array.
Referring again to the general schematic showing of FIG. 1 it will
be seen that the tachometer input is received by a clipper circuit
41 the output of which is connected through a
multiplication-by-four circuit 42 to gate 43. The output of gate 43
is connected both to count modified circuit 44 and to decade
counter 50. The output of count modified circuit 44 is connected to
decade counter 45 which is in turn connected to storage register
46. Storage register 46 is connected to the input of BCD-to-decimal
decoder 47 which is in turn connected to 1-out-of-10 decoder driver
48 which activates the selected light-emitting diode elements of
bar graph display BG.
The output of decade counter 50 is connected to storage register 51
which is connected to BCD-to-seven-segment decoder 52 which
activates the selected light-emitting diodes of numeric display
N.
Intensity modulation unit 55 generates a series of independent
power control pulses which are applied to the light-emitting diodes
of the bar graph display BG via constant current source unit 53,
and to the light-emitting diodes of the numeric display N via
constant voltage (+ fixed resistance) source 54. Brightness pot 56
varies the bias applied to the output of a sawtooth oscillator
circuit (not shown) to control the duration of the power control
pulses and vary the apparent intensity of illumination for both
displays BG and N.
Clock and gate generator circuit 60 is operatively connected in
conventional fashion to gate 43 to control counting of tachometer
input pulses for a precisely-measured predetermined interval. Clock
and gate generator circuit 60 is also operatively connected
conventionally to store registers 46 and 51 to enable them to store
the count results of the decade counters 45 and 50, respectively,
and further operatively connected to decade counters 45 and 50 to
reset them to zero after count results have been stored in the
storage registers 46 and 51.
Normal aircraft power at 28 volts DC is converted to more precisely
controlled 25 volts DC and 5.5 volts DC used for the logic circuits
and activation of the light-emitting diodes in a suitable
arrangement of conventional units 36, 37, 38 and 39.
The tachometer input signal is a sine wave signal received from the
aircraft engine tachometer and has a frequency proportional to
engine RPM. In the preferred embodiment disclosed 100 percent of
rated RPM equals 70 Hz. The input signal is clipped and limited by
clipper circuit 41 and then multiplied by a factor of four in
circuit unit 42 to provide 280 discrete pulses when the engine RPM
= 100 percent rated. his signal passes gate 43 when enabled by a
count signal from the clock and gate generator circuit 60 and is
applied to two channels one including units 50, 51, and 52
associated with the numerics display N and the other including
units 44, 45, 46, 47, and 48 associated with the bar graph display
BG. Count modifier unit 44 is operated alternatively as a
divide-by-five counter of a one-for-one straight-through counter.
When the counter input pulses indicate an RPM of less than 70
percent rated RPM this circuit passes one pulse for each five it
receives. When counter pulses exceed 70 percent rated RPM this
circuit passes each pulse it receives. During the predetermined
interval that gate 43 is enabled by the count signal from the clock
and gate generating means 60 to pass tachometer input pulses the
decade counter 45 and 50 are making the count of these pulses. At
the end of this interval the enabling count signal to gate 43 is
terminated, and the transmission of input pulses therethrough
ceases. The clock and gate generator 60 next transmits a "store"
signal to the storage registers 46 and 51 to enable them to receive
and store the count results from the counters 45 and 50. The stored
count result information in the storage registers 46 and 51 is
simultaneously decoded by the respective decoder units and utilized
to selectively cause energization of appropriate light-emitting
diodes in each of the displays BG and N to indicate visually the
stored count results. Following storage of count results and visual
indication thereof, the clock and gate generator circuit provides a
"reset" signal to each counter 45 and 50 to return the count to
zero and again provides the "count" signal to enable gate 43 to
again pass tachometer input signals to the counters for a
succeeding interval and repetition of the counting and
storage-display cycle.
Referring to FIG. 4, the intensity control system or circuit
embodying principles of the invention is shown, and provides to the
light-emitting diodes a variable duty cycle power signal in the
form of a series of independent square wave power pulses of
variable duration thereby changing the apparent brightness of the
diode display. The output of a sawtooth oscillator consisting of
unijunction transistor T3, resistors R9 and R10, and capacitor C4
as shown in FIG. 4, is divided downwardly by resistors R7 and R8
and applied as one input to a differential comparator T2. The other
input applied to comparator T2 is a DC voltage derived from the
brightness or intensity control pot consisting of resistor R4,
adjustable resistor R5, and resistor R6. Varying this DC voltage
between +6V and +12V causes a variable pulse width output from
comparator T2 which is positive whenever the sawtooth wave exceeds
the DC levels and at ground level when it does not. A feedback
resistor R3 is provided to smooth the overall operation of
comparator T2. The output of comparator T2 is applied to base
resistor R2 and the base of output transistor T1 to complete the
variable duty cycle generator. This generator circuit is biased up
+6V, so that the differential comparator which normally uses
positive and negative voltages can operate from positive supply
voltages only. The output of output transistor T1 is then biased
back down to ground level and 6V by resistor R1 and Zener diode
CR1. The final output stage of the power amplifier consists of
transistor T4, resistors R11 and R12, and capacitor C1 and is the
final driver stage for the display formed by the light-emitting
diodes. Transistor T4 is located toward the front of the instrument
package as shown in FIG. 2. A Zener power supply of +6V and +18V
for differential comparator T2 is shown in FIG. 6 and consists of
resistor R13, Zener diodes CR2 and CR3 and capacitors C2 and
C3.
It is believed to be clear from the above description and
discussion that applicant has provided an intensity control system
for a light-emitting diode display which is a significant
improvement over the prior art systems and achieves the objects of
the invention.
Although a preferred embodiment has been described in detail in
accordance with the Patent Law, many modifications and variations
within the spirit of the invention will occur to those skilled in
the art and all such are considered to fall within the scope of the
following claims.
* * * * *